1. Field of the Invention
This invention relates generally to the fabrication of a MR sensor, which can be a tunneling magnetoresistive (TMR) sensor or a giant magnetoresistive (GMR) sensor. In particular it relates to an MR sensor in which resolution is increased by use of a synthetic antiferromagnetic (SAF) free layer structure.
2. Description of the Related Art
With the ever increasing areal density with which data is stored on magnetic media such as disks in hard disk drives (HDD), the magnetoresistive (MR) sensor that is used as the read-back element in the HDD is required to have a correspondingly improved spatial resolution while achieving and maintaining a reasonable signal-to-noise ratio (SNR). Referring to schematic FIGS. 1a, and 1b, there are shown two views of a generic, prior-art current-perpendicular-to-plane (CPP) tunneling magnetoresistive (TMR) read head.
FIG. 1(A) illustrates the read head in a vertical cross-sectional plane parallel to its air bearing surface (ABS) plane. FIG. 1(B) is a portion of the illustration of FIG. 1(A), isolating the sensor stack portion of the head.
Referring to FIG. 1(A), there is shown the ABS cross-sectional view of the CPP TMR (current perpendicular-to-plane tunneling magnetoresistive) head, in which there is a current that passes perpendicularly to the active magnetic layers through the entire head structure and in which the resistance of the head varies in accord with the physical principles of the tunneling-magnetoresistive effect which is based on the tunneling of conduction electrons through a thin dielectric layer. It is to be noted, however, that the head illustrated could equally well be a giant magnetoresistive (GMR) sensor, which would differ structurally from the TMR sensor in that the thin dielectric layer of the TMR structure would be replaced by a thin conducting layer.
Looking vertically downward, there is first shown an upper (or top) shield (1) that protects the sensor stack (6) from extraneous magnetic fields. At the bottom of the head, there is shown a corresponding lower (or bottom) shield (2) that performs a similar task at the bottom edge of the sensor. Thus the sensor is protected by a pair of shields at some desired separation (3), called the read gap (RG).
Hard bias (HB) magnets (5) (magnets formed of hard, i.e. high coercivity, magnetic material) are laterally disposed to either side of the sensor stack (6). These magnets, which stabilize the magnetization (arrow, 81) of the free layer (8) by a magnetostatic interaction with the free layer, are positioned between the shields (1), (2) and their magnetizations are shown as arrows (51). The sensor stack itself (6) is typically formed as a patterned vertical lamination of horizontal layers, formed beneath an upper capping layer (7).
Referring to FIG. 1(B), there is shown a schematic illustration of the isolated patterned sensor stack (6) of FIG. 1a showing the following vertically stacked horizontal layers patterned to a common longitudinal width: a non-magnetic capping layer (7), the magnetically free layer (8), typically a layer of a NiFe compound, showing its magnetization vector as an arrow (81) that is directed longitudinally (i.e., in a cross-track direction relative to a read operation on a magnetic media); a non-magnetic layer (9) that is a dielectric layer (such as MgO) that serves as a tunneling barrier layer for the TMR sensor (or a conducting layer, such as Cu, for a GMR sensor), a reference layer (10), a coupling layer (eg. a layer of Ru) (11) and a pinned layer (12), coupled to layer (10) across coupling layer (11). The magnetization of layer (12) is held spatially fixed by a thick layer (13) of antiferromagnetic material that pins layer (12). The tri-layer (10), (11) and (12) is referred to as a synthetic-antiferromagnetic (SAF) structure, because even though it is formed of ferromagnetic materials, it has the overall behavior of an antiferromagnetic substance.
The symmetrically, horizontally disposed hard biasing layers (5), each with longitudinal magnetization (51), provides a biasing magnetic field in the sensor stack (6) to orient the magnetization (81) of the free layer (8) in a longitudinal (cross-track) direction by means of a magnetostatic interaction.
In the most modern disk drives, the height at which the head flies above the rotating disk is already less than 5 nm, so the freedom of further flying height reduction to increase spatial resolution is reaching its limit. Thus, the common practice to increase the resolution is by reducing the reader-shield-spacing (RSS) (3) (also denoted RG width), so the magnetic spatial resolution increases correspondingly.
To reduce RSS, the thickness of the hard bias (HB) layers (5) will need to decrease as well. However, reducing the thickness of these magnetic HB layers will also reduce the amount of magnetic “charge” at the edges of the HB layers immediately adjacent to the sensor stack and facing the edges of the free layer. The fewer the magnetic charges, the less is the pinning field of the HB layers and the less effective they are at orienting the free layer magnetization. This, in turn, allows more thermal noise as a result of domain perturbations and a poorer SNR. Finally, reducing the overall thickness of the sensor stack (6) requires reducing the thickness of the horizontal layers that form it, particularly the thick AFM layer (13) that provides pinning for the three-layer structure (10), (11) and (12). Poorer pinning, in turn, will ultimately also adversely affect SNR. Thus, improving resolution by reducing the RG thickness of the sensor is not practical. It is therefore an object of the present invention to achieve the desired improvement in linear downtrack resolution by a method other than the reduction in RG width.
It is to be noted that the prior art discloses methods directed at improving sensor resolution. Examples of such prior art include Carey et al. (US Patent Appl. 2005/0207073) and U.S. Pat. No. 7,580,229, who disclose a hard magnet and exchange biasing for improved resolution; Covington (U.S. Pat. No. 6,809,900) who discloses a SAF free layer with hard magnets; Sakai et al. (U.S. Pat. No. 7,283,377) who discloses an abutted junction exchange biased structure; Sun et al. (US Patent Appl. 2008/0113220) who disclose free and fixed SAF layers and Mao et al. (U.S. Pat. No. 7,016,160), who disclose a plurality of tri-layer readers comprising SAF free layers. However none of these prior art methods describe the present invention.